HYDROGEN ABSORPTION AND DESORPTION-KINETICS OF TIFE0.8NI0.2H

The determination of kinetic reaction rates data must be carried out under strictly controlled heat and mass transfer conditions and related to a well defined reaction controlling mechanism. A number of functions of the reacted fraction, alpha, designated R(alpha), which are characteristic of various reaction mechanisms in the solid state have been presented and successfully applied by Hancock and Sharp [J. Am. Ceram. Soc. 55(2), 74 (1972)]. The integrated rate functions R(cr) reflect the morphology and the geometry of the transforming phases. The functions R(1)(alpha) R(SC)(alpha) and R(D)(alpha) for the first order kinetics, phase boundary migration mechanism known as ''shrinking core'' and diffusion controlled processes, respectively, were shown to be linear in 1/t(0.5) where t(0.5) is the time corresponding to alpha = 0.5. In the present investigation three suitable reaction controlling mechanisms were adapted and modified by introducing a hydrogen pressure dependence function F(P). Isothermal rate measurement experiments for hydrogen absorption/desorption in the two-phase region TiFe0.8Ni0.2-TiFe0.8Ni0.2H in the temperature range 90-150 degrees C yield temperature-dependent rate constants. Pre-exponential factors and activation energies for the first order and the diffusion controlled reaction mechanisms were derived.